Because of the increasing costs of exploration, processing and use of conventional fuels, such as coal, oil and gas, developments have been made to utilize alternative energy sources. One of these alternative energy sources which has been the subject of considerable interest is solar energy. Particularly, vast developments have been made in harnessing and converting solar energy into usable electrical energy.
As a result of the developments made in harnessing solar energy, three primary types of photovoltaic devices have come into being: crystalline solar cells, semicrystalline solar cells, and amorphous solar cells. Each of these solar cell types produces electricity as a result of what is commonly known in the solar cell field as the photovoltaic effect. This is an inherent feature of a semiconductor region in the photovoltaic device which, in the case of amorphous silicon cells, generally comprises adjacent, layered regions of P-type doped amorphous silicon and N-type doped amorphous silicon which define a PN junction, or P-type doped amorphous silicon, intrinsic (I-type) amorphous silicon, and N-type doped amorphous silicon which define a PIN junction. Upon exposure to solar radiation, an electric field is generated across this junction as electrons and holes move thereacross, thereby generating electrical current. For a more detailed discussion regarding the physical structure and electrical characteristics of amorphous solar cells, reference may be made to U.S. Pat. No. 4,409,605 to Ovshinsky et al.
An inherent problem with any type of solar cell involves the collection of electrical energy from the solar cell itself. Particularly, the electrical connection to the surface exposed to solar radiation is of major concern. This is so because any electrical collector means must ideally exhibit good electrical conductivity with high solar radiation transparency. Unfortunately, these two parameters are not found in any one material which is economical to solar cell production. As such it has been necessary to employ at least two distinct electrical conductor means to collect the electrical energy from the solar cell.
One arrangement normally employed in amorphous solar cell design involves a transparent conductive oxide (TCO) layer, such as indium tin oxide or indium oxide, layered over the top layer--either P or N type--of the semiconductor region of the solar cell. The TCO layer permits the influx of solar radiation into the semiconductor region while providing an electrical path therefrom. However, the electrical resistivity of the TCO layer is relatively substantially great and, therefore, restricts electrical current flow over large surface distances. To remedy this problem, it is necessary to provide electrical conductors of lower electrical resistance adjacent to, and electrically communicating with, the TCO layer to receive the electrical current therefrom. Suitable electrical conductors of this type are generally silver ink grids screen-printed onto the TCO surface. However, these grids are limited in size because of their current carrying capacity and the shadowing effect they have on the solar cell, inasmuch as these grids are generally opaque. As such, the overall efficiency of a solar cell declines as the surface area of the grid increases.
One solution to this size limitation problem is to employ several relatively small solar cells electrically joined together in series and in parallel through a network of interconnections to form solar modules and/or arrays. These are capable of providing a variety of voltage and current outputs as a result of the particular electrical interconnections. Another advantage of these solar assemblies is that should any of the individual solar cells fail to produce electrical energy, the entire electrical output of the assembly will only be slightly diminished and generally will remain functional.
The use of multi-celled solar modules, however, is not without inherent problems. Particularly, the interconnections between solar cells are generally relatively small and, as such, are highly susceptible to mechanical and thermal stresses experienced by the solar module. Indeed, the normal handling of the solar module during fabrication, installation, and maintenance may easily damage the electrical connection network. Moreover, the cyclic temperature change which the solar module experiences, as it is exposed to periods of sunlight and periods of darkness, causes thermally induced stresses in the electrical interconnections which ultimately result in fatigue failures thereof.
Efforts to remedy these stress problems have resulted in the general acceptance of electrical interconnections of substantial size which are more resistant to the stress loadings. However, the larger size of the interconnections causes increased shadowing of the solar cells in those areas where they overlay, thereby detrimentally reducing the output of electrical energy from the solar cell. To avoid such optical obscuration losses caused by the shadowing effect on the solar cell, the electrical interconnections in amorphous silicon solar cells, at least, have been joined to the peripheral edge of the TCO layer. However, this, too, causes detrimental reduction in the electrical output of the solar cell as current received by the TCO layer at the distal extreme end from the interconnection junction is required to travel the full width of the solar cell through the poorly conductive TCO layer.
Another problem experienced in these electrical interconnections is the lack of flexibility thereof. Particularly with the development of amorphous solar cells on thin metal substrates, which are substantially more flexible than the crystalline solar cells, the rigidity of the electrical interconnection has been a limiting factor in developing new applications for solar arrays. The desirable characteristic of the solar array of assuming the shape of currently existing support surfaces, an attribute of amorphous solar cells, is substantially precluded because of the inflexible electrical interconnections.
Exemplary of the existing technology pertaining to this aspect of the art are U.S. Pat. Nos. 4,410,558 to Izu et al, and 4,419,530 to Nath. Particularly, Izu et al discloses a system for producing amorphous solar cells in continuous strip form on thin metal substrates. Such a system provides economical solar cell material which may then be employed in solar cell modules and arrays. Nath discloses a method for interconnecting a plurality of solar cells to form a large area solar cell panel. The interconnect system disclosed permits individual solar cells to be selectively electrically isolated from the solar cell panel, as when, for example, an individual solar cell is determined to be defective. While these disclosures constitute advantageous contributions to the art of solar cells, they do not resolve the above-discussed problems.
In addition to the foregoing problems, the process of locating and securing the electrical interconnections relative to each individual solar cell in the array requires substantial man-hours during assembly. This time consuming process results in labor costs which account for a major portion of the overall cost of the solar array. Indeed, the exceedingly high cost of solar arrays has been a considerable hindrance in the development and commercialization of photovoltaic devices.
Despite the substantial work and research directed to development of photovoltaic devices, no photovoltaic cell module or array embodies the desirable attributes currently sought. Namely, no photovoltaic module provides individual solar cells electrically joined together by highly conductive electrical interconnections which are resistant to normally experienced mechanical and thermal stresses. Furthermore, none offers electrical interconnections which avoid shadowing of the solar cell while at the same time providing flexibility at the junctions between individual solar cells.